This invention relates to improvements in line protectors for communications circuits. More particularly the present invention is directed to a plug-in type of module that is used for protecting central office equipment from surge voltages that are applied to the incoming lines, although the invention is adaptable to other types of line protection generally.
Prior art devices for protecting central office equipment are known, for example, by U.S. Pat. No. 3,975,664 that issued Aug. 17, 1976. In devices of that kind protection against surge voltages on either side of the line is provided by a three electrode gas tube in which two of the electrodes are connected respectively to the opposite sides of the line while the third electrode is connected to the ground. The device further includes caps which are in conductive contact with the respective electrodes and are spaced from a grounding clip that is in turn connected to the ground electrode. Spring pressure is applied to these caps tending to force them toward engagement with the grounding clip. In normal operation actual engagement of those caps with the grounding clip is prevented by a solder pellet which melts in an overcurrent condition. Then, the spring pressure is no longer resisted and a direct metallic contact is made between a line electrode and ground.
Another type of surge voltage arrester arrangement is shown in U.S. Pat. No. 3,934,175 that issued Jan. 20, 1976. In that patent, the circuit utilizes a spark gap device as a primary surge arrester and an avalanche semi-conductor element connected in parallel therewith as a secondary surge arrester which serves to quench the energy of the power surge which is not dissipated by the spark gap arrester. The arrangement utilizes a delay line that includes a series connected inductor and a resistor that serves to prevent the avalanche semi-conductor from clamping the spark gap device. The inductor serves to isolate electrically the semi-conductor from the gas tube at relatively fast transient voltage rise times while the resistor provides sufficient voltage drop at slow rise time to limit the damping voltage of the semi-conductor so as to allow the gas tube to fire.
In theory, a surge voltage arrester of the type shown in the aforesaid U.S. Pat. No. 3,975,664 could be used with a separate and distinct unit that includes inductors and semi-conductor elements for each side of the line. Such an arrangement would, however, be relatively cumbersome and would not be suitable for plug-in connection to an existing connector block.